This invention relates generally to a method and system for controlling the reference frequency in a radio receiver. More particularly, this invention relates to a method and system for estimating a frequency offset between a carrier frequency of a transmitter and a local reference frequency of a receiver in a communication system.
FIG. 1A is a block diagram of an exemplary cellular radiotelephone system, including an exemplary base station (BS) 110 and a mobile station (MS) 120. Although denoted a “mobile station”, the station 120 may also be another type of remote station, e.g., a fixed cellular station. The BS includes a control and processing unit 130 which is connected to a mobile switching center (MSC) 140 which in turn is connected to a public switched telephone network (PSTN) (not shown). General aspects of such cellular radiotelephone systems are known in the art. The BS 110 handles a plurality of voice channels through a voice channel transceiver 150, which is controlled by the control and processing unit 130. Also, each BS includes a control channel transceiver 160, which may be capable of handling more than one control channel. The control channel transceiver 160 is controlled by the control and processing unit 130. The control channel transceiver 160 broadcasts control information over the control channel of the BS or cell to mobiles locked to that control channel. It will be understood that the transceivers 150 and 160 can be implemented as a single device, like the voice and control transceiver 170, for use with control and traffic channels that share the same radio carrier.
The MS 120 receives the information broadcast on a control channel at its voice and control channel transceiver 170. Then, the processing unit 180 evaluates the received control channel information, which includes the characteristics of cells that are candidates for the MS to lock on to, and determines on which cell the MS should lock. Advantageously, the received control channel information not only includes absolute information concerning the cell with which it is associated, but also contains relative information concerning other cells proximate to the cell with which the control channel is associated, as described for example in U.S. Pat. No. 5,353,332 to Raith et al., entitled “Method and Apparatus for Communication Control in a Radiotelephone System”.
Modern communication systems, such as a cellular radiotelephone system of the type described above and satellite radio systems, employ various modes of operation (analog, digital, dual mode, etc.) and access techniques such as frequency division multiple access (FDMA), time division multiple access (TDMA), code division multiple access (CDMA), and hybrids of these techniques.
In North America, a digital cellular radiotelephone system using TDMA is called the Digital Advanced Mobile Phone System (D-AMPS), some of the characteristics of which are specified in the TIA/EIA/IS-136 standard published by the Telecommunications Industry Association and Electronic Industries Association (TIA/EIA). Another digital communication system using direct sequence CDMA is specified by the TIA/EIA/IS-95 standard. There are also frequency hopping TDMA and CDMA communication systems, one of which is specified by the EIA SP 3389 standard (PCS 1900). The PCS 1900 standard is an implementation of the GSM system, which is common outside North America, that has been introduced for personal communication services (PCS) systems.
Several proposals for the next generation of digital cellular communication systems are currently under discussion in various standards setting organizations, which include the International Telecommunications Union (ITU), the European Telecommunications Standards Institute (ETSI), and Japan's Association of Radio Industries and Businesses (ARIB).
Direct-sequence (DS) spread-spectrum modulation is commonly used in CDMA systems, in which each information symbol is represented by a number of “chips”. Representing one symbol by many chips gives rise to “spreading”, as the latter typically requires more bandwidth to transmit. The sequence of chips is referred to as the spreading code or signature sequence. At a DS receiver, e.g., a rake receiver, the received signal is despread using a despreading code, which is typically the conjugate of the spreading code. IS-95 and J-STD-008 are examples of DS CDMA standards.
In the mobile radio channel, multi-path is created by reflection of the transmitted signal from obstacles in the environment, e.g., buildings, trees, cars, etc. In general, the mobile radio channel is a time varying multi-path channel due to the relative motion of the structures that create the multi-path.
A characteristic of the multi-path channel is that each path through the channel may have a different phase. For example, if an ideal impulse is transmitted over a multi-path channel, each pulse of the received stream of pulses generally has a different phase from the other received pulses. This can result in signal fading.
When multi-path propagation is present, the amplitude can vary dramatically. Multi-path propagation can also lead to time dispersion, which causes multiple, resolvable echoes of the signal to be received. In the receiver, correlators are aligned with the different echoes. Once the despread values have been weighted, they are summed. This weighting and summing operation is commonly referred to as rake combining.
FIG. 1B illustrates a conventional radio receiver employing a channel estimator. The receiver includes an antenna 10 for receiving signals, a radio receiver 11 for filtering and amplifying signals and converting them to a suitable form for processing, such as complex numerical sample values, a channel estimator 12 which correlates received signal samples with known symbols stored or generated locally in a local code generator 14, to provide channel estimates, and a data decoder 13 for despreading and processing despread signal samples together with the channel estimates to extract information. Data decoder 13 may be, e.g., a rake receiver operating in a manner described in U.S. Pat. No. 5,572,552 to Dent and Bottomley, which is hereby incorporated by reference. A channel estimate updater 15 updates channel estimates based on the latest data and/or pilot symbol decisions and the despread signal samples and provides this updated information to the decoder 13. Decoded data is output for further processing.
Coherent detection requires estimation of how the signals were modified by the transmitter, channel, and/or radio processor. As discussed above, the transmission medium introduces phase and amplitude changes in the signal, as a result of multi-path propagation. The signal may also have become dispersed, giving rise to signal echoes, each echo having a phase and amplitude associated with it, represented by a complex channel coefficient. Each echo also has a delay associated with it. Coherent demodulation requires estimation of these delays and coefficients. Typically, the channel is modeled as discrete rays, with channel coefficients assigned to the different delays.
Channel estimation for a received radio signal using both known modulation symbols embedded in the signal as well as unknown information symbols that are decoded by the receiver is described, for example, in U.S. Pat. No. 5,335,250 to Dent et al., and also in U.S. Pat. No. 5,331,666; No. 5,557,645; and No. 5,619,533 to Dent, all of which are hereby incorporated here by reference. Channel estimation specific to CDMA systems is described in U.S. Pat. Nos. 5,151,919 and 5,218,619 to Dent, which are also hereby incorporated here by reference.
More discussion of smoothing channel estimates using autoregression, that is IIR filtering, may be found in “A Wiener Filtering Approach to the Design of Tracking Algorithms with Applications in Mobile Radio Communications”, Ph.D. thesis of Lars Lindbom, Uppsala University (1995), which is also hereby incorporated by reference herein. This document describes the benefit of adapting a smoothing filter's characteristics to the fading spectrum of the signal.
In the past, the fading spectrum of a signal was assumed to be symmetrical. This is probably true in the long term (i.e., over several minutes), in accordance with Jakes' model for fading in the urban, mobile radio propagation environment. More discussion of Jakes' model and modifications of the model to speed computation during simulations of communication system performance may be found in P. Dent, G. E. Bottomley, and T. Croft, “Jakes' Fading Model Revisited”, Electronics Letters, vol. 29, no. 13, pp. 1162-1163 (Jun. 24, 1993), which is hereby incorporated here by reference.
Jakes' model assumes a uniform angular distribution of reflecting objects around a mobile receiver. The relative Doppler shift of reflected signals arriving at the mobile station at different angles relative to the direction of movement of the mobile station varies with the cosine of the angle of arrival. With a uniform angular distribution, the Doppler spectrum is then symmetrical and two sided, having as much reflected energy arriving from behind the mobile station with a negative Doppler frequency shift as from ahead of the mobile station, having a positive Doppler shift. Rays reaching the mobile station from behind have clearly not propagated an equal distance from transmitter to receiver as rays reaching the mobile station from the front. However, these delay differences have typically been ignored, Jakes' model assuming that rays with such delay differences could nevertheless be combined to produce a net fading waveform for a path of delay equal to the mean of these rays.
More specifically, delays lying within ±0.5 of a modulation symbol period of each other were combined to produce a net fading ray with a mean delay. Delays outside that ±0.5 modulation symbol interval were grouped into a different ±0.5 symbol window to obtain a different net fading waveform with a different mean delay. The different net fading waveforms with their associated modulation-symbol-spaced delays were then taken to characterize a multipath channel, each of the multiple paths nevertheless assuming to conform to Jakes' fading model, i.e., each path is the combination of rays arriving uniformly from all directions.
In a CDMA system, particularly a wideband CDMA (WCDMA) system, chip intervals are much shorter, allowing multiple propagation paths to be resolved with much finer time resolution. Thus, it is no longer valid to use a Jakes' model which adds rays that differ in their propagation delays by even a fraction of a microsecond. This addition was valid only in the context of narrowband FDMA or medium bandwidth TDMA systems. In WCDMA systems, it is necessary to restrict the combination of different rays reaching the receiver to rays that have the same propagation delay from the base station to the mobile station, within ±0.5 of a CDMA chip duration. In the proposed IMT2000 system for next generation mobile telephony which is based on DS-CDMA, a frame has a duration of 10 milliseconds and is divided into 16 slots, each slot being divided into 2560 chips. Depending on the communication channel, 2560 chips are grouped into a number of symbols. For example, in the so-called Perch 1 Channel, there are ten symbols of 256 chips each. A certain number of these symbols are already known and transmitted as pilots from the BS to MSs. One symbol in every paging channel slot is a so-called Perch 2 code. An exemplary CDMA signal format is shown in FIG. 2.
In a 5 MHZ wide WCDMA system, a chip duration is typically 0.25 microsecond (μsec), so ±0.5 chips has a duration of +0.125 μsec, which may be expressed as +37.5 meters in terms of propagation path length variation.
It may be shown that rays with the same delay to this order of accuracy must have reflected from objects lying on an elliptical contour having the base station and the mobile station as its foci. This may be understood with reference to FIG. 3 which depicts elliptical contours representing loci of objects from which the reflection delay is the same. For example, a ray reflected from objects lying on the elliptical contour closest to the mobile station have a delay of T1, rays reflected from objects lying on the next closest elliptical contour have a delay of T2, rays reflected from object lying on the next elliptical contour have a delay of T3, etc.
These objects are not uniformly distributed in angle around the mobile station, nor are they spaced at the same distance from either the mobile station or the base station. Moreover, since the base station lies inside the elliptical contour, if, as is usual, it employs directional transmit antennae, objects around the elliptical contour will not be uniformly illuminated. Consequently, the fading spectrum of a ray of given delay within ±0.5 chip periods will no longer be symmetrical about zero frequency. Rather, the fading spectrum of such a ray will be asymmetrical. This is illustrated in the power spectral plot shown in FIG. 4.
In addition, the offset from zero frequency of the centroid of the fading spectrum is no longer independent of the direction of motion.
One of the purported advantages of WCDMA is that the high time resolution enables resolution of individual reflecting objects such that each resolved ray is a single, non-fading ray, i.e., WCDMA is purported to eliminate fading. Of course, it is recognized that such “non-fading” rays will come and go, but on the relatively longer timescale of lognormal shadowing, which is easier to track. However, each ray will have a varying Doppler spread which means that its phase still varies at up to the Doppler rate, even if its amplitude varies much slower. Thus, there is still the need to track the varying complex value of the propagation channel in order to effect coherent signal decoding, i.e., with knowledge of a phase reference. Moreover, the complete elimination of fading by resolving small reflecting objects is not achieved except using very large bandwidths, beyond the bandwidths of anticipated WCDMA systems, which therefore find themselves in the intermediate region of propagation paths that still each comprise multiple rays. Fading models and channel estimation means for these WCDMA systems are addressed in U.S. patent application Ser. No. 09/227,180 filed Jan. 7, 1999, in the name of Paul Dent, and entitled “Smoothing Channel Estimates by Spectral Estimation”. This application is hereby incorporated here by reference.
A receiver in which the smoothing of channel estimates is adapted separately for per ray channel estimation according to the above-incorporated application is shown in FIG. 5, and channel estimation may be performed as in FIG. 6. The arrangement of FIG. 5 includes a per-ray, asymmetrical smoothing filter synthesizer 20, and a per-ray asymmetrical smoothing filter 21.
In a DS spread-spectrum receiver, a frequency offset or deviation may exist between the transmitter carrier frequency and the local oscillator of the receiver. The frequency offset results from different factors, including temperature variation, aging, and manufacturing tolerances. To address this offset, a phase ramp can be estimated and compensated for in an Automatic Frequency Control (AFC) control loop. Estimation can be based on a pilot channel, pilot symbols, or data symbols with a decision feedback.
In the forward (base-to-remote) link of the IS-95 DS-CDMA system, a pilot channel is available for frequency offset estimation. The pilot is transmitted continuously, allowing tracking of variations in the offset.
It is well known that a receiver AFC provides an estimate of the local crystal reference oscillator's error relative to the remote transmitter, and that this AFC estimate can be used to correct the crystal oscillator in order to correct the local transmitter frequency, as shown in FIG. 7. The arrangement depicted in FIG. 7 includes a frequency error estimator 18. However, past attempts to ascribe observed variations in the received signal in part to Doppler channel variation via a channel estimate and in part to a crystal frequency error were inaccurate, as the channel estimate absorbed part of the frequency error.
These problems are made even worse in a handover situation, in which a mobile station communicating with one base station at a certain frequency offset is handed over to another base station with another frequency offset.
Therefore, there is a need to improve upon the discrimination of signal variations due to channel variations from signal variations due to oscillator error.